Hi Marketo
So you created a Marketo Form...did you put the Form on a Marketo Page or use the embed code on your website?
either way, Marketo will pass the data directly to Marketo!
As for the voucher #, this depends on how you are generating this and how you want to use that data. I suggest looking at the Forms documentation again. You may need to engage a consultant to help you think through this.
also, developers.marketo.com has the technical documents for API.